The specific aims of this study are the evaluation of FK-506 plus 6-mercaptopurine in the therapy of pediatric Crohn's disease unresponsive to steroid therapy and to evaluate if usage of FK-506 brings a more rapid improvement in diarrhea, anemia, and fistula closure than use of 6-mercaptopurine alone. The study will also look at identification of immunologic abnormalities occurring in pediatric patients with Crohn's disease at the systemic and mucosal levels, and the response to immunosuppressive therapy.